An obligate parasite is a parasite that must be with its host, or it dies. Obligate parasites depend on the presence of a host to complete their life cycle. Obligate parasites are common. There are parasitic plants, fungi, bacteria, and animals. The inverse of an obligate parasite is a facultative parasite, a parasite that can complete its life cycle independent of a host.
One of the most standard obligate parasites are viruses. Viruses are bits of genetic material covered in a protein sheath, capable of hijacking the protein synthesis machinery of cells and using them to pump out copies of the virus. Because of their inability to reproduce independently, viruses have sometimes been excluded from the kingdom of life, although this definition of "life" may be inappropriate because there are a number of more complex obligate parasites. Defending themselves from viruses may have been one of the first evolutionary imperatives of bacteria and eukaryotes, and both have evolved a range of error-checking genetic machinery and response mechanisms to slow down viral invaders.
There are other obligate intracellular parasites aside from viruses. These include bacteria like Chlamydias and Rickettsia, among the smallest viruses with the least complex genomes. The Chlamydia bacterium is responsible for the #1 sexually transmitted disease in the world, chlamydia, which is also the foremost cause of infectious blindness. Because obligate intracellular parasites have no tractable genetic system, and cannot be grown in a conventional artificial nutrient environments, and require a tissue culture, they can be difficult to study. Historically, these bacteria were considered to be organisms somewhere between viruses and bacteria.
Even some protozoa (eukaryotes, cells much more complex than bacteria) are obligate intracellular parasites, notably Plasmodium, at least ten species of which infect humans. These are thought to descend from dinoflagellates, photosynthetic protozoa, which eventually lost their photosynthetic ability as their parasitic lifestyle increased in emphasis. Interestingly, it is thought that mitochondria, the power stations present in every human cell, may have begun their evolutionary path as intracellular parasites, but subsequently became so integrated into the host that they actually became a part of it.